(a) Field of the Invention
The present invention relates to an objective for an endoscope and, more particularly, to an objective for an endoscope wherein an inhomogeneous medium is used.
(b) Description of the Prior Art
A retrofocus type objective as shown in FIG. 1 and disclosed by Japanese published unexamined patent application No. 121547/74 is known as an objective for an endoscope.
In recent years, it is required to make the endoscopes smaller and smaller in the diameter. This means that an objective with an extremely small outer diameter is required necessarily.
For an objective with an extremely small outer diameter, it is very difficult to use a plural number of lenses as in case of said known retrofocus type objective due to the reasons related to assembly. Therefore, it is known to arrange that an objective comprises one positive lens as shown in FIG. 2. However, an objective comprising one positive lens has a disadvantage that aberrations are caused largely and, consequently, the field angle is somewhat limited and cannot be made wide.
To obtain an objective with a small outer diameter which supersedes said known objective comprising one positive lens, it may be considered to use a lens made of an inhomogeneous medium as disclosed by Japanese published examined patent application No. 28061/72 and as shown in FIG. 3. For example, it may be considered to use a graded refractive index lens (hereinafter referred to as a GRIN lens) whose refractive index is expressed by the formula shown below when the refractive index of the central portion of said GRIN lens is represented by reference symbol n.sub.0 and the radial distance from the optical axis is represented by reference symbol r: EQU n.sup.2 (r)=n.sub.0.sup.2 {1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 + . . . }
where, reference symbol g represents a parameter showing the degree of the gradient of the refractive index, and reference symbols h.sub.4, h.sub.6 . . . respectively represent the coefficients of distribution of refractive indices in the terms of the fourth order, sixth order and so on.
However, said GRIN lens has a disadvantage that the field angle is decided by the distribution of refractive indices thereof, and it is impossible to make the field angle wide.